[Docs] Fix minor issues in AdvancedBuilds documentation

This patch modifies the commands under the Bootstrap builds section so
that they include the -DLLVM_ENABLE_PROJECTS flag with the value "clang"
so that the Clang build will actually get picked up. Without this patch
they error out as the CLANG_BOOTSTRAP_PASSTHROUGH variable is processed
in /clang/CMakeLists.txt which isn't picked up without the
LLVM_ENABLE_PROJECTS variable being set appropriately.

This patch also changes any remaining dangling <path to source>
references to <path to source>/llvm to better match the rest of the
file.

Reviewed By: thieta

Differential Revision: https://reviews.llvm.org/D148451

Author: boomanaiden154

llvm/docs/AdvancedBuilds.rst

@@ -41,7 +41,10 @@ CLANG_ENABLE_BOOTSTRAP.
 
 .. code-block:: console
 
-  $ cmake -G Ninja -DCMAKE_BUILD_TYPE=Release -DCLANG_ENABLE_BOOTSTRAP=On <path to source>
+  $ cmake -G Ninja -DCMAKE_BUILD_TYPE=Release \
+      -DCLANG_ENABLE_BOOTSTRAP=On \
+      -DLLVM_ENABLE_PROJECTS="clang" \
+      <path to source>/llvm
   $ ninja stage2
 
 This command itself isn't terribly useful because it assumes default
@@ -55,7 +58,11 @@ CMake option, each variable separated by a ";". As example:
 
 .. code-block:: console
 
-  $ cmake -G Ninja -DCMAKE_BUILD_TYPE=Release -DCLANG_ENABLE_BOOTSTRAP=On -DCLANG_BOOTSTRAP_PASSTHROUGH="CMAKE_INSTALL_PREFIX;CMAKE_VERBOSE_MAKEFILE" <path to source>
+  $ cmake -G Ninja -DCMAKE_BUILD_TYPE=Release \
+      -DCLANG_ENABLE_BOOTSTRAP=On \
+      -DCLANG_BOOTSTRAP_PASSTHROUGH="CMAKE_INSTALL_PREFIX;CMAKE_VERBOSE_MAKEFILE" \
+      -DLLVM_ENABLE_PROJECTS="clang" \
+      <path to source>/llvm
   $ ninja stage2
 
 CMake options starting by ``BOOTSTRAP_`` will be passed only to the stage2 build.
@@ -88,7 +95,7 @@ You can build an Apple Clang compiler using the following commands:
 
 .. code-block:: console
 
-  $ cmake -G Ninja -C <path to source>/clang/cmake/caches/Apple-stage1.cmake <path to source>
+  $ cmake -G Ninja -C <path to source>/clang/cmake/caches/Apple-stage1.cmake <path to source>/llvm
   $ ninja stage2-distribution
 
 This CMake invocation configures the stage1 host compiler, and sets
@@ -268,7 +275,7 @@ following commands:
 
 .. code-block:: console
 
-  $ cmake -G Ninja -C <path to source>/clang/cmake/caches/3-stage.cmake <path to source>
+  $ cmake -G Ninja -C <path to source>/clang/cmake/caches/3-stage.cmake <path to source>/llvm
   $ cmake --build . --target stage3 --parallel
 
 After the build you can compare the stage2 & stage3 compilers.